Examples of constant velocity universal joints include a fixed type constant velocity universal joint which allows only angular displacement and a plunging type constant velocity universal joint which allows not only the angular displacement but also axial displacement. Examples of the fixed type constant velocity universal joint include a Birfield type (BJ) and an undercut free type (UJ), and examples of the plunging type constant velocity universal joint include a double offset type constant velocity universal joint (DOJ) and a cross groove type constant velocity universal joint (LJ).
The fixed type constant velocity universal joint of the BJ type includes an outer race as an outer joint member having an inner spherical surface equiangularly provided with a plurality of track grooves formed along an axial direction, an inner race as an inner joint member having an outer spherical surface equiangularly provided with a plurality of track grooves formed along the axial direction in pairs with the track grooves of the outer race, a plurality of balls interposed between the track grooves of the outer race and the track grooves of the inner race so as to transmit torque, and a cage interposed between the inner spherical surface of the outer race and the outer spherical surface of the inner race so as to hold the balls. The cage includes a plurality of window portions arranged along a circumferential direction and housing the balls.
In constant velocity universal joints for automobiles, for the purpose of securing rigidity, machine-structural carbon steel higher in carbon content than case-hardening steel is used as a material for the outer race (outer joint member). The machine-structural carbon steel of the outer joint member used for the constant velocity universal joints is a hard material, and hence difficult to undergo cold forging. Conversely, in the cold forging in which processes are performed in a glasshouse or the like, deformability of a material is markedly lowered in comparison with hot forging, and deformation resistance becomes markedly higher. Thus, materials to be forged are limited. The deformation resistance means stress required for deformation of the materials. When the deformation resistance is high, a processing force becomes higher, and hence stress acting on a die becomes higher. Thus, abrasion, deformation, and breakage of the die are liable to occur. Further, the deformability means such a property as to be deformed without breakage, and is evaluated based on a limit of crack occurrence at the time of a forging process, that is, a magnitude of a processing rate or distortion
By the way, the above-mentioned outer joint member (outer race) is such a component that requires high mechanical accuracy. Thus, conventionally, there has been such a technique that general cold forging is unavailable with respect to the outer al common sense race as a mechanical component, and hence there has been no idea of performing cold forging thereon. Under the circumstance, a raw material obtained by hot forging undergoes a turning process so as to be formed into a shape approximate to a product shape, and then undergoes a grinding process after heat treatment. Products are formed in this manner. Specifically, a radially-inner spherical surface, a cup-inlet chamfer, and track-inlet chamfers undergo a cutting process, and track grooves and the radially-inner spherical surface undergo a grinding process after heat treatment. Thus, conventionally, the cutting process and the grinding process are used in many cases as post processes of a forging process as described above. As a result, man-hours of the post processes are increased, which leads to an increase in manufacturing cost.
Thus, in recent years, there has been proposed adoption of cold forging so that the number of machining steps is reduced (Patent Literature 1).
Citation List
Patent Literature 1: JP 2002-346688 A